Peel and cut the Yukon Gold potatoes into uniform chunks. Cover with cold, salted water in a large pot. Bring to a boil, then simmer until tender, about 20-25 minutes.

Melt salted butter in a small saucepan, add smashed garlic cloves, chopped sage, thyme, and rosemary. Cook for 2 minutes, then stir in heavy cream and steep off heat for 10 minutes.

Drain the cooked potatoes and return to the pot over low heat. Mash until smooth, then mix in the warmed herb butter and cream mixture, followed by the room-temperature mascarpone.

Season with kosher salt and freshly cracked pepper. Adjust to taste and serve immediately, garnished with extra chopped herbs if desired.